There are several issues reported like this, and I've been trying to troubleshoot and track this intermittently for over a year.

Reddit post https://www.reddit.com/r/MagicArena/comments/10cfu3e/game_wont_launch_using_wifi_android/ describes basically this same issue, and yet no fix, just the workaround of using 5g instead.

It's a real annoyance to have to specifically avoid wifi in order to play mtga on a mobile device at home, because the wifi connection is usually better and it's right there.

I've tried enabling/disabling UPnP, adding port forwards (for TCP 9405, 9505), removing those again.
Adding and removing explicit port forwards changed the speed at which the errors occurred.

I usually see timeout eventually ending in "failed to submit deck" when trying to start a game, but when I had 9405 and 9505 forwarded, I got an error much faster.

Who knows what MTGA's matchmaking network architecture is? P2P leveraging UPnP made sense for a moment, but given it's competitive I think hosted servers may be the approach. But if it's that, I really don't understand why my network layout from my gaming device to MTGA server should matter, as it's an outbound connection to establish the handshake / session.

This problem is vexing, though it is not the end of the world. But I'd really like to see it solved.